1. Field of the Invention
The present invention relates to an optical branching and insertion apparatus in a long-distance optical transmission system such as an undersea cable system.
2. Description of the Related Art
Recent years have seen the advent of optical direct amplification repeaters that make use of optical fibers to which erbium or another such rare earth element has been added (EDF) in long-distance optical transmission systems, such as undersea cable systems. Furthermore, the use of wavelength multiplexing transmission technology, in which light signals of a plurality of wavelengths are multiplexed, has facilitated the transmission of large volumes of information.
Meanwhile, with an undersea cable system or the like, because of the difficulty in finding the place where damage has occurred, such as a broken cable, the method generally adopted is to conduct a search for the damage location from the terminal station side using a C-OTDR (Coherent-Optical Time Domain Reflectometer). This C-OTDR is a measurement apparatus that measures the return time of backscattering (Rayleigh scattering) light of signal light sent from the C-OTDR and advancing through the optical fiber (cable), and measures the distance to the break or other damage location of the cable.
FIGS. 7A and 7B are diagrams illustrating a method for searching for a damage location with a C-OTDR. As shown in FIG. 7A, transmission paths 1a and 1b (optical fibers) are connected to a C-OTDR 30. Repeaters 20 are positioned at specific intervals along the transmission paths 1a and 1b, and inside each repeater 20, a light amplifier 21 such as EDF is provided to the transmission paths 1a and 1b. Each repeater 20 has a by-pass path 22 that connects the transmission path 1a to the transmission path 1b. The signal light sent from the C-OTDR 30 advances through the transmission path 1a, and the backscattering light generated during this time advances through the transmission path 1b after going through the by-pass path 22 provided to the repeater 20, and is received by the C-OTDR 30.
FIG. 7B shows the waveform of the backscattering light received by the C-OTDR 30. This waveform includes information about the intensity(level) and the return time (that is, the distance up to the C-OTDR) of the backscattering light. Therefore, when damage occurs in the cable at point P shown in FIG. 7A, the change in the intensity level of the light is measured, as indicated by the dashed line at the position corresponding to point P in FIG. 7B, so it is possible to measure the distance up to this location.
FIG. 8 is a diagram illustrating the structure of an optical branching and insertion apparatus 10 that is installed in an optical transmission system such as an undersea cable system, and that branches signal light of some wavelengths from signal light in which a plurality of wavelengths have been multiplexed by wavelength multiplexing technology. In FIG. 8, the signal light having a plurality of multiplexed wavelengths is inputted from an transmission path 1 to an optical circulator 11.
The optical circulator 11 comprises a path 11-1 that takes the signal light inputted from the transmission path 1 and outputs it to a fiber grating 12 (discussed below), and a path 11-2 that takes the signal light reflected from the fiber grating 12 and outputs it to a transmission path 2.
The fiber grating 12 is a filter means having characteristics such that only specific wavelengths are reflected, while other wavelengths pass through. Therefore, the signal light with the specified wavelengths out of the wavelengths included in the inputted signal light is reflected, while the signal light of other wavelengths is outputted to a transmission path 4 via a optical isolator 13, a fiber grating 14 (discussed below), and a optical circulator 15.
Meanwhile, as indicated by the dashed line in the figure, the signal light reflected by the fiber grating 12 passes through the path 11-2 of the optical circulator 11 and is guided to the transmission path 2. In this way, only the branched light of the specified wavelengths is guided from among the signal light composed of a plurality of multiplexed wavelengths. The transmission path 2 is connected to a branch terminal station (not shown), and the branched signal light is received by the branch terminal station.
The branch terminal station is connected to a transmission path 3 used for insertion of signal light with the same wavelength as the received branched light into a transmission path 4. In specific terms, the signal light that is sent from the branch terminal station and transmitted through the transmission path 3 is inputted to the optical circulator 15. The optical circulator 15 comprises a path 15-1 that takes the signal light from the transmission path 3 and outputs it to a fiber grating 14 (discussed below), and a path 15-2 that takes the signal light reflected by the fiber grating 14 and outputs it to a transmission path 4.
The fiber grating 14 is a filter means having the same characteristics as the fiber grating 12. Therefore, the signal light inputted from the transmission path 3 is reflected by the fiber grating 14, and is outputted to the transmission path 4 through the path 15-2 of the optical circulator 15.
However, with an optical transmission system in which the above-mentioned optical branching and insertion apparatus 10 is installed, there are locations that cannot be searched for when a search is made for the damage location by the C-OTDR discussed above.
FIGS. 9 to 11 are diagrams of a case in which a search for the damage location of an optical cable is made using the C-OTDR 30 from each terminal station (not shown) in an optical transmission system in which the optical branching and insertion apparatus 10 of FIG. 8 is installed. As discussed above, the C-OTDR 30 is connected to a transmission path for sending and to a transmission path for receiving. Therefore, in FIGS. 9 to 11, the various structural elements described for FIG. 8 above are required for each transmission path. Specifically, in FIGS. 9 to 11, transmission paths 1a and 1b, transmission paths 2a and 2b, transmission paths 3a and 3b, transmission paths 4a and 4b, optical circulators 11a and 11b, and optical circulators 15a and 15b are provided corresponding to the various structural elements of FIG. 8. Fiber grating and optical isolators are also provided for each of the transmission paths, although they are not shown in the figures.
First, FIG. 9 is a diagram of a case in which a search for the damage location is made using the C-OTDR 30 from a terminal station (not shown) connected to the transmission paths 1a and 1b. The C-OTDR 30 is generally able to search for a damage location along a transmission path over which is transmitted the signal light sent by the C-OTDR 30 itself, but cannot search for a damage location along other transmission paths. Therefore, in FIG. 9, the transmission paths searched by the C-OTDR 30 are transmission paths 1a, 2a, and 4a.
Here, the damage locations that cannot be searched for by the C-OTDR 30 are section A of the transmission path 4a and section B of the transmission path 2a, as shown in the figure. The reason for this is that the backscattering light that occurs in section A of the transmission path 4a and section B of the transmission path 2a is hindered in its rearward advance by the optical circulators 15a and 11a inside the optical branching and insertion apparatus 10, and so cannot reach the C-OTDR 30.
FIG. 10 is a diagram of a case in which a search for the damage location is made using the C-OTDR 30 from a terminal station (not shown) connected to the transmission paths 4a and 4b. In this case, the C-OTDR 30 searches for damage locations in the transmission paths 1b, 3b, and 4b. Here, the damage locations that cannot be searched for by the C-OTDR 30 are section C of the transmission path 1b and section D of the transmission path 3b, as shown in the figure. The reason for this is that the backscattering light that occurs in section C of the transmission path 1b and section D of the transmission path 3b is hindered in its rearward advance by the optical circulators 15b and 11b inside the optical branching and insertion apparatus 10, and so cannot reach the C-OTDR 30.
FIG. 11 is a diagram of a case in which a search for the damage location is made using the C-OTDR 30 from a terminal station (not shown) connected to the transmission paths 2a, 2b, 3a, and 3b. In this case, the C-OTDR 30 searches for damage locations in the transmission paths 1b, 2b, and 3a. Here, the damage locations that cannot be searched for by the C-OTDR 30 are section A of the transmission path 4a and section C of the transmission path 1b, as shown in the figure. The reason for this is that the backscattering that occurs in section A of the transmission path 4a and section C of the transmission path 1b is hindered in its rearward advance by the optical circulators 15a and 15b inside the optical branching and insertion apparatus 10, and so cannot reach the C-OTDR 30.
FIG. 12 is a diagram illustrating the sections where the damage location cannot be searched for when a search is made for the damage location using the C-OTDR 30 from the above three terminal stations. Specifically, the sections that cannot be searched are sections of transmission paths 4a, 2a, 1b, and 3b in the direction in which the signal light is outputted from the optical branching and insertion apparatus 10, and are sections up to the repeaters where the signal light outputted from the optical branching and insertion apparatus 10 is next inputted. In specific terms, in the transmission path 4a there is a section A from the optical branching and insertion apparatus 10 to the next repeater 20c, in the transmission path 2a there is a section B from the optical branching and insertion apparatus 10 to the next repeater 20e, in the transmission path 1b there is a section C from the optical branching and insertion apparatus 10 to the next repeater 20b, and in the transmission path 3b there is a section D from the optical branching and insertion apparatus 10 to the next repeater 20e.
FIG. 13 illustrates an example of the signal waveform diagram measured by the C-OTDR 30 in a case when there are sections A, B, C, and D where the damage location cannot be searched for as mentioned above. In more specific terms, FIG. 13 is a signal waveform diagram corresponding to transmission paths 1a and 4a when the C-OTDR 30 is connected to the position shown in FIG. 9, for instance. As shown in the figure, the backscattering light in section A from the optical branching and insertion apparatus 10 to the next repeater 20c in the transmission path 4a is not received, so the signal level corresponding to this section is zero (noise level).
Thus, in the past, a problem was encountered when the optical branching and insertion apparatus 10 was installed in an optical transmission system, in that there were sections where the damage location could not be searched for.